1. Field of the Invention
The present invention generally relates to a microfluidic device. More specifically, the invention relates to a microfluidic device in which a micro flow passage, such as a microchannel, is formed.
2. Description of the Prior Art
In recent years, there is known a technique called integrated chemistry for using a microfluidic device, such as a microchip, wherein a micro flow passage (a fine flow passage) having a width and depth of about tens to two hundreds micrometers is formed in a substrate of a glass or plastic, to utilize the micro flow passage as a fluid passage or a reaction vessel, to integrate a complicated chemical system in the microfluidic device. According to such integrated chemistry, a microchip capable of being used in various tests is called μ-TAS (Total Analytical System) if the use of the microchip is limited to analytical chemistry, and the microchip is called micro reactor if the use of the microchip is limited to a reaction. When any one of various tests (any one or combination of operations and means, such as analysis, measurement, synthesis, decomposition, mixing, molecular transportation, solvent extraction, solid phase extraction, phase separation, phase combination, molecule acquisition, culture, heating and cooling) is carried out, integrated chemistry has advantages that the time to transport diffuse molecules can be short since the space in the microchip is small and that the heat capacity of a liquid phase is very small. Therefore, integrated chemistry is noticed in the technical field wherein a micro space is intended to be utilized for carrying out analysis and chemical synthesis.
As such microfluidic devices, there are known microfluidic devices wherein a micro flow passage having any one of various shapes is formed (see, e.g., Japanese Patent Laid-Open Nos. 2002-1102, 2002-239317 and 2003-220322). As methods for forming a micro flow passage in such a microfluidic device, there are known various methods (see, e.g., Japanese Patent Laid-Open No. 2005-230647).
However, when a fluid is allowed to pass through a micro flow passage in such a microfluidic device, there are some cases where air having stayed in the micro flow passage and/or air generated by a pump or the like forms bubbles in the micro flow passage to interrupt the flow of the fluid in the micro flow passage. Particularly in a microfluidic device wherein a narrow portion (a portion having a small flow passage cross-sectional area) is formed in a part of a micro flow passage by providing a columnar portion (a pillar) or the like for allowing the mixing of fluids, a vital reaction or the like in the micro flow passage, there are some cases where bubbles stay in the narrow portion to interrupt the flow of the fluid.